Bald Eagle
| Scientific Name | Haliaeetus leucocephalus |
| Survival Strategy | Short-distance Migrant/ Active All Winter |
| Arrival in Upper Midwest | January to March |
| Departure | When lakes and rivers freeze |
| Destination | Nearest open water |
| Conservation Status | Least Concern |
Spring
If they have not done so already, adult bald eagles lay two eggs and share incubation duties for 35 days. The young remain in the nest for 10 to 12 weeks before fledging.
Summer
Young eagles continue to be dependent on their parents until they are 18 weeks old and will
remain near the nest area for most of the summer as they develop their hunting skills.
Fall
By early fall, young eagles are independent of their parents. As cold weather begins, eagles will migrate (typically south) only as far as necessary to find open water and food. By late fall, local birds and migrants from more northern regions congregate in large numbers along the ice-free portions of larger rivers in the upper Midwest. Some will remain year-round and feed by hydroelectric dams and on roadkill.
Winter
By mid-January, eagle pairs begin defending their nest sites. Each year, the eagles’ treetop nest, called an aerie, increases in size as the couple performs site maintenance, adding sticks and plant material such as cornstalks and grass. Typically, eagles lay two eggs (occasionally three) between early February and early April.
Behavior
Winter Feeding
In cold weather, eagles will conserve energy by staying as inactive as possible, preferably perched somewhere that blocks the wind and offers a good view of potential feeding opportunities, such as the trees lining the river banks near hydroelectric dams. Without needing to use much energy, these skilled hunters will swoop down and grab any fish that comes to the surface. By quickly filling their crop with fresh fish, they reduce the chances of other eagles stealing their food.
While these winter predators provide an excellent opportunity for bird watching and nature photography, it is essential to remember that whether you are watching bald eagles, rough-legged hawks, or snowy owls, finding food is a matter of survival for them. If humans get close enough to cause the birds to fly, they will burn precious calories and possibly miss out on their next meal. Consider watching from a distance and staying in your car (it will be warmer for both you and the birds).
Physiology
The Value of a Warm Coat
Incubating fragile eggs when the brutal cold of an early spring blizzard covers the nest with six inches of snow seems like a daunting undertaking, but the bald eagle’s two-layered coat is up to the task. Like the outer shell worn by winter enthusiasts, the eagle’s contour feathers serve as a raincoat and a wind block. An inner layer of down feathers creates tiny air spaces that retain body heat. By fluffing their contour feathers (piloerection), the bird can trap more air, increasing the insulation value of its down feathers and reducing the number of calories needed to survive the cold.
Countercurrent Heat Exchange
Bald eagles use countercurrent heat exchange to conserve heat they would otherwise lose through their exposed legs. The veins and arteries in the bird’s featherless legs are close to each other. Before the warm arterial blood gets to the eagle’s feet, the veins absorb much of the heat and return it to the body. This exchange of heat keeps a bird’s feet colder, often just above freezing. Birds have little muscle or nerve tissue in their exposed feet, so there is no need to waste energy keeping them warm
Saving Energy
In cold weather, bald eagles conserve life-sustaining energy by lowering their body temperature at night. By “turning down the thermostat” just a little, eagles can reduce their daily energy consumption by over 4%, enough to improve their chances of fending off hypothermia and surviving through the winter night.
Diet
The bald eagle’s diet is primarily fish and includes waterfowl and carrion. If given a chance, they are opportunist carnivores and steal prey from other animals (kleptoparasitism).
Lifespan and Mortality
The median lifespan for bald eagles is 16.5 years, with some living 30 years. About 50% of eaglets survive their first year, and only 10% survive to adulthood (5 years).
Predators
Raccoons will sometimes kill young in the nest, as will the occasional great horned owl.
Siblings
The first year of life is the most lethal for bald eagles. Eagles lay 2 to 3 eggs over a 3-to-6-day period and begin incubating immediately. The first eaglet to hatch will be larger and more dominant during feeding than its siblings. A clutch with three eggs puts the last to hatch at a tremendous size and strength disadvantage, especially in times of food scarcity. Due to starvation, hypothermia, or aggressive siblings (siblicide), survival of the third eaglet is rare.
Human Interactions
Feeding on roadkill carrion can result in death by collision with cars. Ingesting lead-shot from deer hunters’ gut piles can result in lead poisoning.
Climate Vulnerability
The Audubon Society models predict a 74% reduction in bald eagle habitat by 2080.
Never Stop Learning
Etymology: Bald eagles get their misleading name from the adjective “piebald,” meaning “having a combination of black and white coloring.”
First Nations: The Potawatomi term for the bald eagle is, “mgéshwash.”
References
| Buehler, D. A. (2020). Bald Eagle (Haliaeetus leucocephalus), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.baleag.01 |
| Journey North: Bald Eagle https://journeynorth.org/eagle/ |
| Siciliano Martina, L. 2013. “Haliaeetus leucocephalus” (On-line), Animal Diversity Web. Accessed January 15, 2022 at https://animaldiversity.org/accounts/Haliaeetus_leucocephalus/ |
| Eldon Greij explains why some birds practice siblicide https://www.birdwatchingdaily.com/news/science/eldon-greij-explains-siblicide/ |
| The Raptor Resource Project https://www.raptorresource.org/ |
| Did Benjamin Franklin really want the turkey to be the US national bird? https://www.livescience.com/benjamin-franklin-turkey-national-bird |
| Potawatomi-English dictionary: mgéshwash (bald eagle) https://glosbe.com/pot/en/mg%C3%A9shwash |